Hi,
Some folks call this thing a "tickler" (not sure why). Anyway, it's kinda like the choke bubble on a lawnmower. Press it and it shoots a little gas into the carb to help get it started.
Charlie
Here's how it works. It's simply a small pin held up by a spring. When you push it down the end of the pin contacts the float, pushing it down slightly. This opens the needle valve and lets the float bowl overflow into the venturi if you hold it down long enough.